Single Chip Modules (SCMs) and Multi-Chip Modules (MCMs) used to package Integrated Circuit chips (ICs) for a variety of electronics applications are well known in the art. In applications where significant amounts of heat are generated during operation of SCM or MCM devices, secondary external cooling hardware must be attached to the modules to provide optimum performance and reliability during operation. To ensure development of low thermal resistance pathways at interfaces between modules and heatsinks, a common method for cooling SCMs and MCMs is to attach aluminum heatsinks to module surfaces, module lids, or directly to surfaces of one or more of the chips present on the modules. In any of these configurations, optimum cooling efficiency and desired mechanical properties of the module to heatsink interface are provided by using a thin layer of low thermal resistance, thermally conductive, compliant adhesive to bond the heatsink to a given module or device surface or surfaces on the module. In this specification, the term compliant is defined as the ability of an adhesive to absorb or take up mechanical stresses between the adhesive and the bonded surfaces.
However, in many applications, including the use of SCMs and MCMs in high performance computers, both modules and interconnecting printed circuit boards (PCBs) are very expensive, and can be fairly restricted in supply. As a consequence of these expense and supply constraints, attached heatsinks used for improved cooling performance must also be removable from either module or chip surfaces, so that repair and reuse of the PCBs, modules, and heatsinks is possible. In order to facilitate heatsink removal to accommodate subsequent electronics hardware reuse and repair, it is desirable to bond heatsinks to modules using a thermally conductive adhesive that softens upon local heating of module and module site on the PCB. After external heat is applied, removal of the heatsink is achieved by using both applied torque (twisting) and vertical tensile (pulling) forces. Simultaneous application of these forces results in "taffy pull" stretching and eventual cohesive or interfacial bond failure of the adhesive, and allows for complete separation of the heatsink from the module. After the heatsink is removed, any residual adhesive on module, chip, or heatsink bond surfaces is removed with either chemical or mechanical methods, thus allowing the heatsink, the module, and the circuit board to be either repaired or reclaimed for potential reuse.
Unfortunately, in certain applications, use of the aforementioned adhesive heatsink attach structure and rework methodology is inadequate. For example, when large heat dissipation requirements are necessary, the intrinsic thermal conductivity of the attach adhesive limits the application thickness of the adhesive that can be used to provide a low thermal resistance interface that effectively transfers heat generated from the operating module to the heatsink. In this case, control of variables that affect both the ability to apply a thin adhesive bond line and the ability to control the resultant thickness of the attach adhesive after the heatsink is placed in contact with the module become critical to ensure in-service reliability of the module IC devices. If the adhesive layer is applied too thick, or is too thick or inconsistent in thickness after heatsink attach, overheating can result and prompt latent in-service module IC device failures. Potential for excess adhesive thickness is most pronounced in packaging configurations where a single heatsink is directly attached to multiple chip surfaces of a MCM. For this application, both flatness of IC chip attach surfaces and relative planarity of the module IC chips with respect to one another as attached on the MCM module surface are critical to ensure both identical and minimal adhesive layer thickness results at the interfaces between all IC chips and the bond surface of the heatsink after it has been attached to the module. However, since planarity of the chips cannot be guaranteed to less than 0.010 inches without use of costly alternative finishing methods such as mechanical IC chip backside lapping, the potential for both excessive and significantly inconsistent adhesive interface bond layer thicknesses can exist at one or more IC device locations. Therefore, as shown in FIG. 1, a separate heatsink 11 must be used on each chip 13 of module 15 since the outer surfaces of chips 13 are not co-planar, and to provide thin adhesive layers of consistent thickness.
Although adhesive thickness and thickness variation can be minimized by using independent multiple heatsinks on MCMs, the presence of multiple heatsinks in package designs drives other problems when used with the prior art heatsink design and attach methodology. These problems can include both heat dissipation uniformity resulting from heatsink placement variation on the module, and heatsink removal constraints imposed by the geometry of the heatsinks required for the packaging application. To ensure uniform heat dissipation, the heatsinks must be in very close proximity to provide consistent module coverage and heatsink cooling fin spacing. However, when independent heatsinks having square or rectangular base geometries are required to be uniformly close to one another, mechanical interference between adjacent heatsinks can occur during the removal operation. This mechanical interference inhibits the ability to apply adequate torque to remove the individual heatsinks from the modules. Thus, the use of separate heatsinks does not allow for completely effective module rework and removal when used in conjunction with the prior art attach configuration.
In other applications, such as the section of PCB assembly shown in FIG. 2, a PCB 21 has multiple IC modules 23 on both of its surfaces or sides. Card 21 requires both individual heatsinks 25a, that provide heat dissipation for individual modules, and individual array heatsinks 25b that provide for cooling of banks or arrays of two or more modules 23 that are clustered in closely spaced groups on the card surfaces. Modules on both sides of the PCB require heatsinks to adequately cool the hardware and ensure reliability. Heatsinks 25b and their modules 23 are typically held together with retention pins 27 which extend through large holes in card 21. Heatsinks 25b provide cooling of non-planar module surfaces through the use of a compliant, thermally conductive pad 29 affixed between each heatsink 25b and their modules 23. In addition, heatsinks 25b may be removed for rework by removing pins 27.
Although heatsinks 25a, 25b of in FIG. 2 are easily removed, the design has three disadvantages. First, space required for the retention pin holes in the card compromise card wireability. Second, the thermal pads offer inferior thermal performance compared to direct bond conductive adhesives. Finally, the dual-side array heatsink assembly is relatively expensive.
To overcome these shortcomings, attempts have been made to bond large array heatsinks to the module banks using the previously described conductive adhesive bonding approach for direct heatsink attachment. Unfortunately, heating the card and heatsink assembly followed by "taffy pull" heatsink removal delaminates the copper module attach pads on the PCB. In some cases, entire banks of modules and corresponding copper attach pads on the PCB can be inadvertently removed from the card surfaces. Thus, adhesive bonding approaches do not consistently support card rework.
An alternative interconnect method for reworking and replacing wirebonded chips attached to carrier surfaces is described in U.S. Pat. Nos. 5,601,675 and 5769,989. The method may be generally summarized as follows and is depicted in FIG. 3. A fusible release layer 31 such as solder is applied between a chip carrier pad 39 and a wirebond technology chip 35 present on a carrier surface 33. The chip 35 is bonded to release layer 31 with a layer of adhesive 37.
With this method, rigid adhesives that are in general not effectively reworkable by thermal softening or chemical or mechanical means are used to ensure high yield wirebonding. Local heat is applied immediately around chip 35 to melt the release layer 31 beneath the chip bond adhesive 37, thereby allowing for clean separation of both chip 35 and adhesive 37 from the chip carrier pad 39 on the carrier surface 33. A new chip is then bonded at the vacant site with fresh adhesive, followed by curing and subsequent electrical attachment. The release layer has a melting temperature which is compatible with wirebond process temperature requirements and other reflow assembly operations to ensure structural stability. A modified version of this method could be used to removably secure heatsinks to IC modules.